This invention relates to an abnormality determination system for revolution speed sensors, which determines if either of two speed sensors, one built in an input shaft and the other in an output shaft of an automatic transmission, is abnormal.
Shift control of an electronically-controlled automatic transmission is performed based on a vehicle speed, a throttle valve opening and other driving conditions. A vehicle speed is especially important as the basis of shift control. Thus, if an incorrect speed signal is outputted, there is a possibility that the automatic shift control cannot be accurately performed. For that reason, an abnormality determination system for revolution speed sensors has been proposed, which determines if either of the speed sensors, one built in an input shaft and the other in an output shaft of an automatic transmission, is abnormal by comparing outputs from both of the speed sensors, and which prevents the automatic transmission from using outputs from the abnormal speed sensor for shift control.
The abnormality determination processing described above is executed by comparing the number of output pulses from a first speed sensor built in an output shaft of a transmission with that from a second speed sensor built in an input shaft of the transmission. For example, while a predetermined number of pulses are outputted from the second speed sensor, if no pulse is outputted from the first speed sensor, the first speed sensor is determined to be abnormal. On the other hand, while a predetermined number of pulses are outputted from the first speed sensor, if no pulse is outputted from the second speed sensor, the second speed sensor is determined to be abnormal.